As office equipment such as computers, word processors and copying machines advance, a growing number of printing apparatus for outputting information from these equipment has become available on the market. The printing apparatus employing an ink jet printing system in particular has an advantage of being able to reduce the size of a print head easily, print an image at high resolution and high speed and print on plain paper without requiring special processing on the paper. Other advantages include low running cost, low noise and a relative ease with which a full color printing can be realized using multiple color inks. It has therefore found a wide range of applications, including personal users.
Such a widespread use can lead to the user making new demands on the ink jet printing apparatus. In recent years in particular, there are growing calls for increased image fastness such as waterfastness and lightfastness while maintaining a high color saturation. One method of enhancing the image fastness is to make some improvements on the print medium as dedicated paper. However, to stably maintain a high image fastness of various kinds of print mediums including plain paper, it is more effective to provide an ink itself with some features to achieve the above objective. For this reason, recent years have seen many novel inks developed and their applications proposed.
For example, Japanese Patent Application Laid-open No. 11-227229 discloses, in addition to the conventionally used dye inks, the development of inks containing pigments as coloring materials and a variety of printing methods using such inks. The inks containing pigments tend to stay on the surface of a print medium with the colorants in a coagulated state, when compared with inks containing dyes as colorants. Thus, the pigment colorants have features of a high color saturation which is not easily faded by sunlight and ozone. To take advantage of both the superiority of the pigment ink and the superiority of the dye ink, the above-cited reference discloses a method that selectively uses these different kinds of inks according to the kind of print medium used and the kind of image to be output. For example, the above document describes that a pigment-based black ink with low penetrability and dye-based color inks with high penetrability are prepared and that a black image may be printed with the black ink or with a combination of different color inks, depending on the kind of print medium and the kind of image to be printed. The cited reference also describes printing color inks first, followed by a black ink overlapping the first printed color inks.
Other methods for enhancing the color saturation and the image fastness propose using a reaction liquid that reacts with color inks containing colorants to make the colorants insoluble or coagulate. For example, Japanese Patent Application Laid-open No. 56-89595 discloses a method which applies a polymer solution, such as carboxymethyl cellulose, polyvinyl alcohol and polyvinyl acetate, to the print medium before printing and then prints coloring inks. Japanese Patent Application Laid-open No. 63-299971 discloses a method that involves applying to a print medium a liquid containing an organic compound having two or more cationic groups in one molecule and then printing coloring inks containing anionic dye. Japanese Patent Application Laid-open No. 64-9279 discloses a method that first applies an acidic liquid containing succinic acid to a print medium and then prints coloring inks. Japanese Patent Application Laid-open No. 64-63185 describes a method that applies to a print medium a liquid that makes a dye insoluble, before printing coloring inks containing the dye. Japanese Patent Application Laid-open No. 5-202328 describes a method which applies a reaction liquid containing polyvalent metal ion before printing coloring inks.
Further, Japanese Patent Application Laid-open Nos. 6-106841, 9-118850, 11-334101 and 11-343441, and U.S. Pat. Nos. 5,428,383, 5,488,402 and 5,976,230 disclose a set of a black ink and color inks in which at least one of the color inks exhibits a mutual reactivity with the black ink, with other inks showing no reactivity with the black ink.
All these methods listed above that use a reaction liquid are characterized in that the reaction liquid chemically reacts with the coloring inks containing colorants to coagulate the coloring inks. That is, many ink jet printing apparatus of recent years, whether they use pigments or dyes or whether they require a reaction liquid to induce coagulation, have the colorants coagulate, remain and settle on the surface of a print medium, thereby realizing a satisfactory color saturation and image fastness.
There are also growing calls for a high image quality and an ease of handling that match those of silver salt pictures. In recent years, an increasing number of printing apparatus are appearing on the market which can perform the so-called “marginless printing” by which an image is printed to the edges of the print medium.
In the conventional ink jet printing apparatus, forming an image to the edges of the print medium poses many problems to the apparatus. One of the problems is that ink that has overrun the edges of the print medium may contaminate the interior of the printing apparatus, further contaminating sheets of print medium as they are fed into the printing apparatus. Since the ink is absorbed also at the edges of the print medium, the accuracy with which the print medium is transported degrades, which is likely to result in the print medium being jammed in the apparatus.
However, a construction and method to solve the above problems accompanying the “marginless printing” have already been proposed, for example, in Japanese Patent Application Laid-open Nos. 10-128964 and 2000-351205. As a construction to realize the “marginless printing” on side edges of a print medium, Japanese Patent Application Laid-open No. 10-128964 discloses an “ink jet printing apparatus which comprises: a guide means set movable, according to the size of the print medium, in a direction perpendicular to the direction of transport of the print medium and installed inside of the side edges of the print medium; and an ink receiving means installed outside of and adjacent to the guide means in a direction perpendicular to the print medium transport direction to receive ink from the print head.” That is, when the “marginless printing” is performed on print mediums of various widths, ink ejected outside of the side edges of the print medium can be received by the ink receiving means, thereby minimizing the contamination of the interior of the printing apparatus.
Japanese Patent Application Laid-open No. 2000-351205 discloses a construction to realize the “marginless printing” with respect to front and rear ends of a print medium. In this construction, a platen surface that restricts the position of the print medium during printing is formed with a hole and ink ejected outside the front or rear ends of the print medium during the printing operation is led into the hole, in which an absorbent is installed to absorb wasted ink. The mechanism to collect ink ejected outside the edges of the print medium without contaminating the interior of the apparatus is one of the important factors in realizing the “marginless printing.”
In the ink jet printing apparatus, however, it is found that the use of the above inks, though it can improve the image quality on a print medium, may pose a new problem in terms of the handling of ink in the printing apparatus. One such example will be explained as follows.
Since inks are handled in the ink jet printing apparatus, a material for absorbing ink is installed inside the apparatus in a variety of forms. A waste ink absorber for example is used to absorb ink discharged from a print head in a recovery operation of the print head. Other examples include a preliminary ejection pad that accepts ink droplets ejected from the print head for stabilizing its ejection performance and an ink absorber that accepts ink droplets ejected outside the edges of the print medium during the “marginless printing.” The absorbers installed at various locations in the apparatus are supposed to be able to absorb ink quickly and the material for the ink absorber is chosen based on this requirement. The quick and reliable absorption can prevent ink droplets from being scattered inside the apparatus.
However, if an ink with a coagulating property such as described above is used, a quick absorption of ink as with common dye inks becomes difficult to achieve. Such an ink has colorants not dissolved in water and ionized as with dyes but dispersed in a liquid, so when it adheres to the absorbent, it is not absorbed as quickly as water. The phenomenon and problems that the inventors of this invention have found in the process of executing the “marginless printing” using pigment inks as an example of coagulating inks and also dye inks will be explained as follows.
FIG. 13 shows a dye ink as it is ejected onto an ink absorber. In the figure, denoted 1 is a print head. Ink ejected from the print head 1 is a conventionally known water-based dye ink for use in ink jet printing. The dye used may include water-soluble dyes such as a direct dye, an acid dye and a basic dye. Denoted 2 is an ink absorber which may use any type of commonly known porous material. The ink absorber may be formed, for example, by using fibers of cellulose, rayon, acrylic, polyurethane or polyester singly or in combination and forming these fibers into fibrils or by subjecting the fibers to a hydrophilic surface treatment and laminating them in layers. The ink absorber may also be formed of porous polyethylene and melamine foam. If such an ink absorber 2 is used in combination with the dye ink, the ink will quickly be absorbed in the ink absorber, with the ink soaking into the interior of the ink absorber 2 as shown shaded in the figure.
FIG. 14 shows a pigment ink as it is ejected onto an ink absorber similar to the above. Any conventionally known pigment ink for use in ink jet printing may be used. In a combination of such a pigment ink and the ink absorber, a part of ink components such as liquid medium penetrates into the ink absorber 2. However, the pigment particles remain on the ink absorber 2 forming a deposit as an ink component left unabsorbed. That is, as shown shaded in the figure, the ink separates into a portion that penetrates into the absorber and a portion that deposits on the ink absorber and settles there.
While in the above explanation a pigment ink has been taken for example, such an ink behavior in the absorber can similarly be observed in any ink with a coagulating colorant. For example, the same also applies even to an ink composed of a mixture of dye and pigment in which the pigment constitutes a main colorant with another colorant such as a highly soluble dye mixed with it for color adjustment. The similar effect can also be produced even when a dye is used as a colorant, by using a reaction liquid that reacts with the dye to accelerate the coagulation of the colorant.
In the absorber and the preliminary ejection pad during the execution of the “marginless printing”, the ink deposit on the surface of the absorber progressively increases as the number of printed sheets and the power-on time increase. Once the surface of the absorber is covered with the deposit, ink droplets landing on the absorber thereafter fail to be received in the absorber. As a result, ink bounced off the absorber surface will contaminate the interior of the printing apparatus. Further, when a large number of sheets are “marginless-printed”, it is found that the ink deposit reaches the print medium transport path, contaminating the back of the print medium. Furthermore, it is also found that the ink deposit may protrude even into the print medium transport path, touching the end of the print medium, which in turn may result in a transport failure.
In the case of a waste ink absorber, the transported ink coagulates on the surface or in the interior of the absorber, inhibiting a smooth transport of ink.
FIG. 15A and FIG. 15B are schematic diagrams showing how the above problem occurs. What is shown here is an example case that uses inks and a reaction liquid that reacts with these inks to coagulate colorants. In the figures the waste ink of K, C, M, Y is transported through a tube 1521 to the waste ink absorber 1531 where it is absorbed. The reaction liquid, on the other hand, is absorbed through a tube 1522 into the same waste ink absorber 1531. The two kinds of waste liquids are mixed inside the waste ink absorber 1531 inducing a chemical reaction. Therefore, where the two waste liquids meet, an area 1553 is formed in which ink becomes stagnant because of coagulates and insoluble substances produced by the reaction. This stagnant area 1553 is small at an early stage of use of the apparatus as shown in FIG. 15A but as the number of printing operations and suction-based recovery operations increases, the region progressively expands as shown in FIG. 15B, eventually blocking an ink flow path into the waste ink absorber 1531, which in turn will adversely affect the ink suction performance during the recovery operation. If an absorber of a large capacity is prepared, the liquids are not soaked uniformly into the interior of the absorber, degrading an absorption performance and therefore an ink accommodating capacity of the absorber.
Such a problem with the waste ink absorber surfaces more or less when a pigment ink is used even if a reaction liquid is not used, because in terms of coagulating the colorant the use of the pigment ink produces the similar effect to that of the case where the reaction liquid is used.
As described above, in an ink jet printing apparatus using a coagulating ink for an improved image quality, ink absorbers installed at various locations in the apparatus have been found to have a degraded absorbing capability and unable to perform their intended function.